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On August 2 Marin County Open Space District sponsored an update meeting on the evolution of Bolinas Lagoon as part of the ongoing study of what action(s), if any, need to be taken to preserve the lagoon as a healthy tidal estuary.
Members of the "project reformulation" consulting team made presentations to the advisory group (Bill Carmen, Josh Collins, Gwen Heistand and Gary Page) and to the pubic. Roger Byrne (UC, Berkeley) discussed his recent sediment core analyses and Don Danmeier (Philip Williams Associates) presented findings regarding the lagoon's physical changes since the 1850's. Mike Josselyn (Wetlands Research Associates) and Phil Williams (PWA) were on hand to participate in the "question and answer" session that followed the presentations.
Although the analyses will not be completed until later this month, a number of interesting conclusions were presented. Two that stood out are
- Most of the sediment that has accumulated in Bolinas Lagoon in the past 150 years comes from the ocean, not the watershed, and
- The bottom of the north lagoon area dropped up to four feet in the 1906 earthquake, not one foot as previously believed.
The 1906 earthquake was a "double-edged sword" while it increased the lagoon's tidal prism, there is evidence that the Bolinas Bluffs collapsed and the collapsed material became a source of sediment to enter the lagoon. In addition, the deepened portion of the lagoon became a sediment trap that contributed to an increased rate of sedimentation.
Annual sediment accretion has been more than twice the tidal prism gained from sea level rise, which suggests that, barring intervention, at some future time the lagoon's inlet may close. The duration of the closure will depend, in part, on the time of year in which it occurs (wet or dry season). The consulting team is continuing to analyze data that will yield conclusions regarding the likelihood of inlet closure within the next fifty years. The consulting team is also analyzing the impact of the Pine Gulch Creek Delta on water circulation in the north lagoon.
The current work schedule calls for the analyses to be completed this month, and a draft report submitted to the project "peer review group." A public presentation of the study conclusions, and distribution of the report, will be made in September. In October, a meeting will be held for the public to comment on, and question, the report's findings, and in November the public will meet with Marin Open Space District and County officials to determine whether or not intervention is called for, and if so, in what time frame.
Project bulletins, correspondence and status updates continue to be available at the MCOSD’s web site: www.marinopenspace.org.
Bucky Mace
For the Foundation Board